In recent years, with the request for speeding-up of signal transmission in electronic parts and electronic equipments containing such a part therein, there has been a demand for development of those low in electric resistance of wiring between electrodes as circuit boards for fabricating LSI packages such as BGA and CSP or circuit boards, on which these semiconductor devices are mounted. Therefore, it is extremely important to measure the electric resistance of wiring between electrodes of such a circuit board with high precision in electrical inspection of the circuit board.
In the measurement of an electric resistance of a circuit board, there has heretofore been adopted, for example, the four probe method, in which probes PA and PD for current supply and probes PB and PC for voltage measurement are pressed against and brought into respective contact with 2 electrodes 91 and 92 to be inspected, which have been electrically connected to each other in a circuit board 90 to be inspected, as illustrated in FIG. 30, a current is supplied between the probes PA and PD for current supply from a power supply device 93 in this state, and a voltage signal detected by the probes PB and PC for voltage measurement at this time is processed in an electric signal processor 94, thereby finding an electric resistance value between the electrodes 91 and 92 to be inspected.
In the method described above, however, the surfaces of the electrodes 91 and 92 to be inspected may be damaged by pressing the probes PA and PD for current supply and the probes PB and PC for voltage measurement against them because it is necessary to bring the probes into respective contact with the electrodes 91 and 92 to be inspected by considerably high pressing force, the probes are made of a metal, and the tips thereof are pointed. It is therefore impossible to use such a circuit board. Under such circumstances, the measurement of the electric resistance cannot be performed on all circuit boards to be provided as products, and so the method cannot but adopt the so-called sampling inspection. After all, the yield of the products cannot be increased.
In order to solve such a problem, there have heretofore been proposed electric resistance-measuring apparatus, in which connecting members coming into contact with electrodes to be inspected are made up of a conductive elastomer.
For example, (i) Patent Art. 1 discloses an electric resistance-measuring apparatus, in which elastic connecting members formed of conductive rubber obtained by binding conductive particles with an elastomer are arranged at electrodes for current supply and electrodes for voltage measurement, respectively, (ii) Patent Art. 2 discloses an electric resistance-measuring apparatus having a common elastic connecting member provided so as to come into contact with surfaces of both electrode for current supply and electrode for voltage measurement electrically connected to the same electrode to be inspected and formed of an anisotropically conductive elastomer, and (iii) Patent Art. 3 discloses an electric resistance-measuring apparatus comprising a circuit board for inspection, on the surface of which a plurality of inspection electrodes have been formed, and an elastic connecting member provided on the surface of the circuit board for inspection and formed of a conductive elastomer, wherein two electrodes among the inspection electrodes are selected in a state that electrodes to be inspected have been electrically connected to the plurality of inspection electrodes through the connecting member, one of both electrodes is used as an electrode for current supply, and the other is used as an electrode for voltage measurement to measure an electric resistance.
According to such electric resistance-measuring apparatus, the electrodes for current supply and the electrodes for voltage measurement are brought into contact with electrodes to be inspected of a circuit board to be inspected through the elastic connecting member, whereby electrical connection is achieved, so that the measurement of the electric resistance can be conducted without damaging the electrodes to be inspected.
However, the measurement of an electric resistance between electrodes by the electric resistance-measuring apparatus of the constructions (i) and (ii) involve the following problems.
In recent years, the size and pitch of electrodes or an interelectrode distance in a circuit board have tended to become small for the purpose of achieving a high degree of integration. In the electric resistance-measuring apparatus of the constructions (i) and (ii), however, it is necessary to electrically connect both electrodes for current supply and electrodes for voltage measurement at the same time to electrodes to be inspected in a circuit board to be inspected, the electric resistance of which should be measured, through the elastic connecting member, respectively. In the electric resistance-measurement apparatus for measuring an electric resistance as to a circuit board to be inspected, on which small-sized electrodes to be inspected have been arranged at a high density, it is thus necessary to form an electrode for current supply and an electrode for voltage measurement corresponding to each of the small-sized electrodes to be inspected in a state separated from each other in a region of an area equal to or smaller than a region occupied by the electrode to be inspected, i.e., to form an electrode for current supply and an electrode for voltage measurement, which are smaller in size than the electrode to be inspected, in a state separated from each other at an extremely short distance.
On the other hand, in order to improve productivity, it is adopted as a production process of circuit boards to produce a combined circuit board composed of a plurality of circuit boards linked with each other by a board material, collectively conduct electrical inspection as to the respective circuit boards in the combined circuit board in this state and then cut the combined circuit board, thereby producing a plurality of circuit board separated from each other.
However, the area of the combined circuit board, which is an object of inspection, is considerably large, and the number of electrodes to be inspected is also extremely great. When multi-layer circuit boards are produced in particular, the number of steps in the production process thereof increases, and the number of times subjected to thermal hysteresis by a heat treatment increases, so that electrodes to be inspected are often formed in a state misregistered from the prescribed arrangement positions. When measurement of an electric resistance between electrodes is performed as to the circuit board to be inspected, which is large in area and has a great number of electrodes to be inspected formed in a state misregistered from the prescribed arrangement positions as described above, by the electric resistance-measuring apparatus of the constructions (i) and (ii), it is extremely difficult to electrically connect both electrode for current supply and electrode for voltage measurement at the same time to each of the electrodes to be inspected.
Description is given by a specific example. When an electric resistance as to an electrode T to be inspected having a diameter L of 300 μm is measured as shown in FIG. 31, a clearance D between an electrode A for current supply and an electrode V for voltage measurement to be electrically connected to the electrode T to be inspected is about 150 μm. When the position of the electrode T to be inspected to the electrode A for current supply and the electrode V for voltage measurement deviates from the prescribed position shown in FIG. 31 by 75 μm in a direction arranging the electrode A for current supply and the electrode V for voltage measurement in alignment of a circuit board to be inspected as shown in FIGS. 32(a) and 32(b), electrical connection between any one of the electrode A for current supply and the electrode V for voltage measurement and the electrode T to be inspected is not achieved, and so necessary measurement of the electric resistance cannot be conducted.
It is considered as a means for solving such a problem to lessen the clearance D between the electrode A for current supply and the electrode V for voltage measurement to, for example, at most 100 μm. However, it is extremely difficult in fact to produce such an electric resistance-measuring apparatus.
On the other hand, according to the electric resistance-measuring apparatus of the construction (iii), there is no need to form an electrode for current supply and an electrode for voltage measurement corresponding to each of electrodes to be inspected, so that tolerance for positional deviation to a circuit board to be inspected, the electric resistance of which should be measured, becomes high even when the circuit board to be inspected is large in area and has a great number of electrodes to be inspected, and the electrodes to be inspected are small in size and arranged at a high density. In addition, such an electric resistance-measuring apparatus is easily produced.
Since such an electric resistance-measuring apparatus is a measuring apparatus according to the so-called pseudo-four-probe method, however, a measurement error range is great, and it is thus difficult to perform measurement of an electric resistance as to a circuit board low in electric resistance between electrodes with high precision.
In order to solve such a problem, there has been proposed a connector for measurement of electric resistance, which comprises an insulating substrate, and plural pairs of connecting electrodes formed on a front surface of the insulating substrate and each composed of a core electrode and a ring-like electrode arranged so as to surround the core electrode (see Patent Art. 4).
According to such a connector for measurement of electric resistance, at least a part of the ring-like electrode is located on an electrode to be inspected in a circuit board, the electric resistance of which should be measured, so far as alignment is made in such a manner that at least a part of the core electrode is located on the electrode to be inspected. Accordingly, electrical connection of both core electrode and ring-like electrode to the electrode to be inspected is surely achieved even when the circuit board is large in area and has a great number of small-sized electrodes to be inspected, so that measurement of an electric resistance as to the circuit board can be surely performed with high precision by using any one of the core electrode and the ring-like electrode as an electrode for current supply and the other as an electrode for voltage measurement.
However, the above-described connector for measurement of electric resistance involves a problem that the whole structure thereof is complicated to encounter difficulties in producing it at a high yield.    Patent Art. 1: Japanese Patent Application Laid-Open No. 26446/1997    Patent Art. 2: Japanese Patent Application Laid-Open No. 2000-74965    Patent Art. 3: Japanese Patent Application Laid-Open No. 2000-241485    Patent Art. 4: Japanese Patent Application Laid-Open No. 2003-322665